I have a 2 ton split AC that would draw about 12A current and is rated for 2260W input power at 230V. With this much information how do I choose a solid state relay? Since the Rating is in inductive what do I look for?

  • \$\begingroup\$ That's going to be a big, expensive solid state relay. \$\endgroup\$ – Matt Young Oct 10 '15 at 15:43
  • \$\begingroup\$ Your looking for a relay rated for 12A motor loads. Any thing rated for resistive or general purpose loads is going to weld the contacts (for normal relays) or blow the solid state switch. \$\endgroup\$ – Spoon Oct 10 '15 at 16:28
  • \$\begingroup\$ You may want a motor relay or contactor. "Contactors" are really relays, but they are specifically designed for controlling power to AC motors. Personally I would not go solid state for this load, but if you do, pay attention to power dissipation in the relay. \$\endgroup\$ – mkeith Oct 10 '15 at 16:32
  • \$\begingroup\$ Yep!... this uk.rs-online.com/web/p/contactors/3927380/…-FeaturedProductsContent--3927380 is sort of what you need. \$\endgroup\$ – Spoon Oct 10 '15 at 16:40
  • \$\begingroup\$ No tell me what do I need to check? The parameters? I just don't understand it when "inductive" loads come into picture. Everything becomes expensive and hotter. When they specify 2260W power can't a relay thats rated 20A@230V be sufficient , isn't Power = V X I = 230 X 20 =4600W, which is much higher than required? If this how its not calculated, how do I actually calculate? \$\endgroup\$ – Rakshith G B Oct 10 '15 at 18:37

Nothing special is needed. You can buy a SSR without zero-cross detector for rated current, let's say 25A or more. Parallel at the output you should install aditional snubber network, that is a resitor and capacitor in series. Zero-cross SSR isn't a good choice for inductive loads, while a circuit withoust snubber can have problems while turning off.



About different types of SSR and their applications: http://www.crydom.com/en/tech/newsletters/solid%20statements%20-%20ssrs%20switching%20types.pdf

  • \$\begingroup\$ Wait, how is the SSR controlled if the zero-cross detector is removed? And you've linked me to a 40A SSR with 24-380VAC , how did you choose this? \$\endgroup\$ – Rakshith G B Oct 11 '15 at 15:27
  • \$\begingroup\$ I have just searched the ebay, it doesn't mean it is the right for you, just an example. You would need to get the datasheet and make sure is random turn on (non zero cross). Random turn on, turns on at no matter what is the actual phase voltage. \$\endgroup\$ – Marko Buršič Oct 11 '15 at 17:30
  • \$\begingroup\$ One thing I understood now is "random turn on" SSRs are used for inductive loads. So thats done. Now how do I check for the rating? Since my AC runs at 12A, can I go for a 20A random turn on SSR? \$\endgroup\$ – Rakshith G B Oct 11 '15 at 18:37
  • \$\begingroup\$ Yes, look what you get for the price, usually a SSR brick is rated 25A, 40A, 65A,.. also look if it has in built snubber, otherwise you can buy it or make it with a 0.1uF/250VAC cap + 100 ohm/1W resistor. Also find a suitable brick for your input control voltage if it is AC or DC. \$\endgroup\$ – Marko Buršič Oct 14 '15 at 16:39
  • \$\begingroup\$ Finding a random turn on SSR is a real pain. Why is it so rare? \$\endgroup\$ – Rakshith G B Oct 15 '15 at 16:31

Inductive loads are hard on relays during turnoff.Relays that that are specified for motors are available and have a HORSEPOWER rating as well as a voltage and current rating .When the relay states a motor horsepower rating you cant go wrong .These good relays are more expensive and are physicaly larger and have generous contact spacing and are often referred to as contactors .You can go solid state if you know what you are doing .For safety there should be a mechanical contact in series with the SSR contact .For disipation reasons there should be a contact in paralell with SSR contact.The SSR aproach has been shown to be more complicated than the simple mechanical approach .The SSR way may have to be considered if your job involves a large number of switching operations.Fortunately the manufacturers of mechanical switchgear state number of operations Vs current at inductive loads .

  • \$\begingroup\$ This still doesn't explain what I asked for. Assuming my AC is rated at 2HP , you mean to say I should look for a "contactor" rather than a relay that is rated well above 2HP? Aren't relays an exact replica of mechanical switches? If the switches can handle the "sparking" during turn off why can't the SSRs? Even if I opt for a SSR instead of contactor, what parameters am I looking for? I did try digging for a SSR that also mentions Inductive Loads or HORSEPOWER, sadly no one seems to give that rating. So how do I work with a SSR? \$\endgroup\$ – Rakshith G B Oct 11 '15 at 6:50
  • \$\begingroup\$ SSR's have several problems with motor applications... the primary one being that they almost never turn completely off. Contactors generally remain the best way of switching motor loads, and run for usually millions of cycles. The contacts are better than plain relays, they have arc deflectors to quench the arc on opening, and a much stronger pull-in and drop out than normal relays, again, to reduce and contain arcing. \$\endgroup\$ – R Drast Oct 12 '15 at 10:59

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